Hexavalent chromium compounds have been used as traditional conversion coatings to treat metal surfaces to improve their corrosion resistance and paint adhesion. Hexavalent chromium shows toxicological effects and has been determined by the Environmental Protection Agency as a risk to the environment and by the Occupational Safety and Health Agency as a health risk. Moreover, chemistries based on hexavalent chromium are classified as carcinogens by these agencies.
It is highly desirable to provide coatings and processes free of hexavalent chromium but still capable of providing paint adhesion and corrosion resistance comparable to that provided by conventional hexavalent chromium-based coatings to metal surfaces. Within the past few decades, various compositions and processes not relying on hexavalent chromium have been described and used for treating metal surfaces. Some of these are based on at least partial replacement of hexavalent chromium with trivalent chromium.
Known uses of partially reduced chromium, i.e. compositions containing some trivalent chromium and some hexavalent chromium, include acidulated rinses for phosphated metal surfaces. These acidulated rinse solutions may contain a partially reduced hexavalent chromium solution, i.e. a solution wherein some of the chromium is trivalent. Partially reduced chromium solutions may be manufactured by adding an organic compound such as formaldehyde, methanol, ethanol, etc. to a solution of chromic acid. The chromic acid oxidizes the organic compound, and is itself reduced in the process. The organic compound(s) must generally be added very carefully and slowly to the chromic acid solution, because the resultant heat, boiling and generation of CO2 can be so energetic as to be dangerous. Yet these materials are seldom taken to more than 30% reduction of chromium VI to chromium III unless excess mineral acid is added to help hold the trivalent chromium in solution. Efforts to more completely reduce the chromium VI typically result in sludge formation, and full conversion to trivalent chromium is not achieved by such methods. An exemplary partially reduced (typically about 30% reduced) chromic acid is available from Bulk Chemicals, Inc. of Reading, Pa. under the trade name BULK RINSE® 6A.
The incomplete conversion of chromium VI to chromium III in these and other processes is undesirable in some applications, and thus, methods of producing fully reduced chromium salts would be of benefit to the industry.